The Gáe Bulg (also Gáe Bulga, Gáe Bolg, Gáe Bolga, meaning "notched spear", "belly spear", "bellows-dart," or possibly "lightning spear") was the spear of Cúchulainn in the Ulster Cycle of Irish mythology. It was given to him by his teacher, the warrior princess Scáthach, and its technique was taught only to him.

It was made from the bone of a sea monster, the Coinchenn, that had died while fighting another sea monster, the Curruid. Although some sources make it out to be simply a particularly deadly spear, others - notably the Book of Leinster - state that it could only be used under very specialized, ritual conditions:

The Gáe Bulg had to be made ready for use on a stream and cast from the fork of the toes. It entered a man's body with a single wound, like a javelin, then opened into thirty barbs. Only by cutting away the flesh could it be taken from that man's body.
- The Táin, trans. Thomas Kinsella

In other versions of the legend, the spear had seven heads, each with seven barbs. In the Táin Bó Cuailnge, Cúchulainn received the spear after training with the great warrior master Scáthach in Alba. She taught him and his foster-brother, Ferdiad, everything the same, except she only taught the Gáe Bulg feat to Cuchulainn. He later used it in single combat against Ferdiad. They were fighting in a ford, and Ferdiad had the upper hand; Cúchulainn's charioteer, Láeg, floated the Gáe Bulg down the stream to his master, who cast it into Ferdiad's body, piercing the warrior's armor and "coursing through the highways and byways of his body so that every single joint filled with barbs." Needless to say, Ferdiad died soon after. On a separate occasion, Cúchulainn also killed his own son, Connla, with the spear. In both instances, it was used a last resort, as once thrown it proved invariably fatal.

Copied straight from it's wiki entry. I just thought I would mention this one incase it hasn't been already.

They say it's made from the bone of a sea monster and I was wondering it's from stingray. Though I'm not sure if the the 'Coincehnenn' ( It may be something more related to the giant squid.) and the StingRay
are one and the same.

Heres a picture of the actual barb of stingray. This is what may have been attached to the end of the spaer shaft in the forging of weapon only used by the honoured Celtic warriors who were taught the art of 'Spearach Caden'.